It is known that a number of fuel cells are joined together to form a fuel cell stack. Such a stack generally provides electrical current in response to electrochemically converting hydrogen and oxygen into water. The electrical current generated in such a process is used to drive various devices in a vehicle or other such apparatus. A supply generally provides hydrogen to an anode side of the fuel cell stack. The fuel cell stack may use less hydrogen than provided by the supply to generate electrical power. A mixing chamber (or ejector) receives unused hydrogen discharged from the fuel cell stack and combines the unused hydrogen with the hydrogen generated from the supply to sustain a flow of hydrogen to the fuel cell stack. In some cases, the unused hydrogen may include impurities such as water that is in the form of de-ionized water (DI) vapor and/or nitrogen which may need to be removed from the unused hydrogen prior to the delivery of the unused hydrogen to the ejector. The impurities generally result from the use of air, rather than pure oxygen. The nitrogen, from air, may cross over into the unused hydrogen by diffusion through a membrane in the fuel cell from a cathode side. A majority of the water (both in liquid and in vapor form) is discharged from the fuel cell stack to an exhaust of the cathode side on the fuel cell stack. However, a fraction of the water generated may permeate into the unused hydrogen. The mixing chamber presents the supply hydrogen with the unused hydrogen to the fuel cell stack. The recirculation of the unused hydrogen to the fuel cell stack may improve fuel efficiency.
The amount of flow of hydrogen that is passed through the fuel cell stack depends on the amount of current generated by the fuel cell stack. In a high current generating mode, the flow of the unused hydrogen discharged from the fuel cell stack is generally high since the fuel cell stack has to consume more hydrogen at a relatively faster rate in order to generate a greater amount of power. In a low current generating mode, the flow of the unused hydrogen discharged from the fuel cell stack is low since the fuel cell stack consumes a small amount of hydrogen while producing smaller amounts of power.
In the low current generating mode, the low flow rate of unused hydrogen may make it difficult to remove impurities and cause fuel starvation since the water generated in response to electro-chemically combining hydrogen and oxygen diffuses to an anode side and without sufficient gas flow to push the water droplets, catalyst sites on the membrane electrode assembly (MEA) are blocked. The low flow rate of the unused hydrogen and the presence of impurities in the unused hydrogen may affect the production of electrical power and adversely affect the life span of the fuel cell stack.